1. Field of the Invention
The present invention relates to the measurement of the rate of permeation of a gas or vapour or a mixture of a gas and a vapour through a sample of a material (polymer, metal, ceramic material, composite, semiconductor, biological material or a combination thereof).
2. Description of the Related Art
US 2003/0001086A1
US 2002/0173922A1
U.S. Pat. No. 5,390,530
JP63132137
EP 1373861
Nörenberg, H. et al.: Review of Scientific Instruments 70 (1999) 2414-2420.
Permeation is the transmission of molecules and atoms through samples. Measuring the rate of permeation of vapours and gases through materials, in particular barrier materials, is important in various fields. Examples include packaging of food, medical supplies, electronic components and components in fuel cells and fuel tanks. Barrier layers serve the purpose of preventing or restricting the passing of a gas or vapour.
A number of methods are known to measure the rate of permeation using infrared sensors, electrochemical sensors, optical sensors or pressure sensors. Mass spectrometric methods, methods using a calcium layer (“Calcium test”) and methods using radioactive tracers are known too.
The known applications using infrared, electrochemical, total pressure or optical sensors are limited with respect to their sensitivity. The Calcium test is limited to species, which undergo a chemical reaction with the calcium layer. Radioactive tracer methods are limited to a very restricted number of radioactive isotopes available, whose handling is very cumbersome and potentially dangerous due to their radioactive nature.
In some known mass spectrometric application a gas cell is filled with an amount of liquid and then introduced into a vacuum chamber. After some time a saturated atmosphere is established inside a gas cell, which contains vapour at the partial pressure of the liquid. In the case of water and water vapour a relative humidity of 100% is established inside the gas cell. This method creates a number of difficulties and limitations.
The vapour pressure of a vapour in an enclosed volume above its liquid phase is unequivocally linked to its temperature by thermodynamic law. Consequently, the vapour pressure generated above a liquid phase in equilibrium and the temperature cannot be varied independently. This unequivocal link between temperature and vapour pressure means, that at different temperatures the said vapour pressure is different. The vapour pressure of water vapour varies exponentially between about 2340 Pa at a temperature of 20° C. (293K) and 101300 Pa at a temperature of 100° C. (373K). This represents a 43-fold increase in the water vapour pressure at a less than twofold increase of the absolute temperature.
It is certainly desirable to test samples under conditions, which they may encounter in practical use. For instance mobile phones containing organic light-emitting (OLED) displays will be used in different climates. Therefore testing the display under hot and humid conditions as well as testing under hot and dry conditions is desirable.
It is an disadvantage of the known method that the temperature cannot be varied at a given water vapour pressure or that at a given temperature the water vapour pressure cannot be varied. A further disadvantage is, that the known method does not allow compensating for the increased pressure at elevated temperatures, which may cause undesirable mechanical stress, which may damage the sample.
In known applications a gas cell is filled with a gas. The signal measured with a mass spectrometer decays as the gas cell is depleted of gas caused by permeation. The decay of the signal can be approximated by exponential functions to extract the rate of permeation. This method is impractical, cumbersome and inaccurate, as the exponential parameters have to be determined for each permeating species.
In known applications the sample is mounted flat in a gas cell. This limits the sample to simple shapes such as sheet material. More complicated structures containing materials of different permeability, including complete electronic devices such as batteries, or sub-assemblies of devices including their edges, are impossible to test in this arrangement
Most conventional methods measure the average rate of permeation of the sample. The disadvantage is that they do not provide information about whether the permeation through the sample is homogeneous or varies across the sample due to defects, other inhomogeneities or areas of different permeability of the sample, which are put there on purpose.